1. Field of the Invention
The invention relates generally to strain gauges and more particularly to an electrically operated strain gauge providing an extraordinarily high sensitivity to applied stress.
2. Description of the Prior Art
It has long been a goal to develop a strain gauge of compact design that is highly sensitive. U.S. Pat. No. 4,884,453 to Hoffmann et al. for example relates to a ferromagnetic thin film type of strain gauge possessing magnetostrictive and anisotropic magnetoresistive properties. According to the disclosure in the Hoffmann et al. patent, magnetostrictive behavior causes a change in the direction of magnetization away from an easy axis whenever the strain gauge is subjected to stress. In proportion to the mechanical stress applied, the magnetization changes its direction by as much as 90.degree.. This change in direction yields a corresponding change in the resistance of the thin film of less than 2% due to the anisotropic magnetoresistance effect.
Research activity in the last few years involving magnetic multilayer assemblies has been described in scientific literature, unrelated however to magnetostrictive types of strain gauges. Generally, such magnetic multilayer assemblies comprise alternating thin film layers of ferromagnetic material and nonferromagnetic conductive material. Magnetic moments of the magnetic fields in alternate layers of the ferromagnetic material naturally align in a perfect 180.degree. antiferromagnetic coupling arrangement parallel to an easy axis. Upon introduction of an external magnetic field, the magnetic moments realign by relative angular displacement up to 90.degree.. Such realignment results in a corresponding change in electrical resistance due to what is now known as a "spin-valve" effect. Resistance changes of as high as 65% have been reported at room temperature.
The foregoing referred to "spin-valve" effect depends not on the rotation of a single magnetic moment and the resulting change in the direction of magnetization, but on the realignment of antiferromagnetically coupled magnetic moments. Such resistance changes are proportional to the degree of realignment.
It is an important object of the present invention to provide a strain gauge of a multi-film layer type that is readily adaptable to miniaturization.
It is a further object of the present invention to provide a strain gauge that can be miniaturized for operation on microchips or integrated into other small devices.